1. Field of Invention
This invention relates to ceramic dielectric insulating materials. More specifically, it relates to a ceramic material having a high dielectric constant with low temperature dependence and low energy loss, for use in the manufacture of compact ceramic capacitors. It further relates to a method of making such a ceramic material.
A high dielectric constant is essential for the manufacture of compact ceramic capacitors. For a given desired capacitance, other properties being equal, the size of a capacitor is inversely proportional to the dielectric constant. A high dielectric constant thus allows a given capacitance to be provided in a small component.
Another property of importance in the manufacture of a capacitor is the temperature dependence of the dielectric constant. The use of a material with a dielectric constant which is relatively constant over a wide temperature range is desirable because a capacitor using such dielectric material can function in an electrical system over a wide operating temperature range without appreciable variation in capacitance.
The Electronic Industry Association has designated stable capacitance characteristics in which the variation of dielectric constant with temperature over a specified temperature range is within certain limits. The designation "Negative Positive Zero" (NPO) refers to dielectric materials with a dielectric constant less than 150 and no more than 1% variation in dielectric constant between -55 and +125 degrees C. The designation Y7R for what are referred to as "stable mid-K" class II dielectrics, stands for a range of dielectric constant from 600 to 4000, with a maximum of +15% variation in dielectric constant between -30 and +125 degrees C.
Another important property in the choice of dielectric materials for ceramic capacitors is the heat dissipation, which is related to electrical conductivity. This property is commonly expressed in terms of "tangent delta", a factor which is directly proportional to the power dissipated as heat due to current flow across the ohmic resistance of the material when an alternating voltage having a frequency of 1 to 90 kHz is applied across the dielectric material. A value of less than 0.03, or 3%, in terms of tangent delta is considered to be satisfactory.
Yet another important factor from the standpoint of capacitor manufacture is the reproducibility of dielectric properties from batch to batch of ceramic material being prepared. This reproducibility relies on the proper control of the crystal size in the ceramic dielectric material during its preparation.
Dense, fine-grained, fired ceramic materials, especially barium titanate-based materials, with high dielectric constants exhibiting low temperature dependence and low heat dissipation have long been sought. However, it has proved difficult to find materials meeting all the above criteria. The admixture of metal oxides and the manner of processing, such as milling, firing temperatures, time or atmosphere under which the material is fired, have been found to affect the homogeneity, grain size, and other properties of the ceramic dielectric material.
Conventional methods of incorporating metal oxides into the base material for grain growth control generally produce materials of uneven, coarse grains. Frequently, such materials exhibit appreciable variations in dielectric constant with temperature and pronounced peaks in dielectric constant at certain temperature within the normal operating temperature range of electronic circuit components.
2. Description of Prior Art
U.S. Pat. No. 4,468,472 discloses ceramic dielectrics containing oxides of Ba, Ce, Zr, a and Ti with the optional oxides of Mn, Fe, Ni, and Co. The oxide mixture on sintering forms crystals below 5 microns in size. The dielectric constant exhibits negative temperature dependence. The dissipation factor is above or equal to 1.0%. U.S. Pat. No. 4,058,404 discloses a sintered ceramic dielectric containing strontium titanate, barium titanate, bismuth oxide, and titanium oxide exhibiting low changes in dielectric constant with temperature and low dissipation loss.
U.S. Pat. No. 3,775,142 discloses an NPO-type of ceramic material having high titanium oxide content (60-65 mol %) with oxides of Ca, Sn and Zr ranging 0-5 mol %. A low dielectric constant in the range of 55-90 was achieved. Canadian Patent 920,348 discloses a ceramic dielectric having a ceramic portion, consisting of oxides of titanium and zirconium and barium titanate; and a glass portion consisting of oxides of cadium, silicon and boron. The dielectric constant is in the range of 55-90.
Molokhia and Issa (Pramana, Vol. 11, No. 3, Sept. 1978, pp. 289-293) attempted to improve the dielectric properties of a barium titanate ceramic by adding zirconia. The dielectric constant of the ceramic showed strong temperature dependence.